For some users, when they add one ticket using the selector and click to proceed, they receive an error: “Nothing in your event queue. You need to select at least one event before you can proceed with the registration process.”
A user that has this occurring is using Windows 10, happened on both Chrome and Edge (updated versions).
I checked, and both of the sites listed under your developer support license are hosted on WPEngine. You’ll need to contact WPEngine support and ask them to add exclusions for Event Espresso’s checkout pages. There’s more information about this in the documentation: